Conventionally, wireless communication systems have been known that adopt an Orthogonal Frequency Division Multiplexing (hereinafter, referred to as “OFDM”) transmission scheme. OFDM is one of multicarrier modulation schemes, and features large tolerance to multipath fading occurring when the propagation path is complicated by obstacles, as compared with the conventional single-carrier modulation scheme.
In recent years, a one-cell reuse OFDM system has been proposed which is aimed to apply high-speed technique by OFDM to mobile terminals such as cellular telephones and the like. In such a one-cell reuse OFDM system, a Multilevel Transmit Power Control (hereinafter, referred to as “MTPC”) scheme is known as a technique for providing tolerance to interference with other cells. This MTPC scheme is to use an adaptive modulation scheme for transmitting a subcarrier causing large attenuation of the reception power by multipath fading using a modulation scheme with a low modulation level, while transmitting a subcarrier causing small attenuation of the reception power using a modulation scheme with a high modulation level, and adjust the transmit power of a subcarrier for transmitting data so as to obtain a desired SNR, and has received attention as measures against multipath fading from the viewpoints of limitations in maximum value of the transmit power and the like, and efficient use of subcarriers.
Described herein are differences in transmission spectrum between the typical OFDM system and the OFDM system applying MTPC. FIG. 15 contains graphs to explain the differences in transmission spectrum between the typical OFDM system and the OFDM system applying MTPC. FIG. 15(a) shows a transmission spectrum in the typical OFDM system, and FIG. 15(b) shows a transmission spectrum in the OFDM system applying MTPC. In FIGS. 15(a) and 15(b), the vertical axis represents the transmit power, and the horizontal axis represents a subcarrier number.
It is assumed in FIG. 15 that the number of subcarriers is twelve, and that the subcarrier number is assigned sequentially from the left. It is further assumed that FIG. 15 (b) shows a case of using four modulation schemes, 64QAM (Quadrature Amplitude Modulation), 16QAM, QPSK (Quadrature Phase Shift Keying) and BPSK (Binary Phase Shift Keying), and shading is provided according to the modulation scheme. Dots are provided in the case that the modulation scheme is 64QAM, oblique lines are provided in the case of 16QAM, horizontal lines are provided in the case of QPSK, and shading is not provided in the case of BPSK. Such fading is the same as in explanatory drawings of the transmission spectrum in this specification as described below.
As shown in FIG. 15 (a), in the typical OFDM scheme, modulation is performed using the same modulation scheme in all the subcarriers, and data is transmitted with equal transmit power in all the subcarriers. In contrast thereto, in the OFDM system applying MTPC, as shown in FIG. 15(b), modulation is performed using a modulation scheme with a different modulation level for each subcarrier corresponding to the propagation path state, and the transmit power is controlled for each subcarrier. More specifically, (1) a subcarrier with a good propagation path state is modulated using a modulation scheme with a high modulation level, while a subcarrier with a poor propagation path state is modulated using a modulation scheme with a low modulation level;    (2) the transmit power of each subcarrier is adjusted corresponding to quality of the transmission path so as to obtain a desired reception SNR on the receiving side for each subcarrier; and    (3) a subcarrier with extremely low quality of the propagation path is not provided with the transmit power, and set as a carrier hole.
FIG. 15(b) shows the case where subcarrier number 1 is set for 64QAM as a modulation scheme, subcarrier numbers 2 and 3 are set for 16QAM, subcarrier numbers 4 to 6, 11 and 12 are set for QPSK, subcarrier numbers 7, 8 and 10 are set for BPSK, and subcarrier number 9 is set for a carrier hole.
FIG. 16 is a diagram showing a configuration example of a wireless communication system for performing communication using such an OFDM system applying MTPC. The wireless communication system as shown in FIG. 16 is assumed to be comprised of a one-cell reuse cellular system where the same carrier frequencies are used in all cells.
As shown in FIG. 16, a base station apparatus 20 is installed in each cell 10, and performs bidirectional communication with a mobile station apparatus (hereinafter, referred to as a “terminal” as appropriate) 30. It is assumed in FIG. 16 that the OFDM system applying MTPC is used in transmission (downlink) from the base station apparatus 20 to mobile station apparatus 30. A communication scheme of transmission (uplink) from the mobile station apparatus 30 to base station apparatus 20 and the frame format are not particularly limited, and it is possible to use known communication schemes and frame formats.
The mobile station apparatus 30 receives a downlink signal transmitted from the base station apparatus 20, analyzes the signal, and estimates an interfering power level received from peripheral cells. Then, the apparatus 30 notifies the base station apparatus 20 of the estimated interfering power level. Based on the interfering power level notified from the mobile station apparatus 30, the base station apparatus 20 sets a modulation scheme, transmit power and carrier hole for each subcarrier.
In the wireless communication system as shown in FIG. 16, FIG. 17 is a diagram showing structures of frames transmitting to mobile station apparatuses 30 in cells 10 from base station apparatuses 20, respectively. FIG. 17 shows structures of frames transmitted to terminals A, B and C among the mobile station apparatuses as shown in FIG. 16.
As shown in FIG. 17, it is assumed that transmission timing of an OFDM symbol to each terminal from the base station apparatus 20 is synchronized among the calls. Shown herein is the case where at time t1 is started transmission of a first OFDM symbol to the terminal A, at time t2 is started transmission of a second OFDM symbol to the terminal A, and transmission of a first OFDM symbol to the terminal B, and at time t3 is started transmission of a third OFDM symbol to the terminal A, transmission of a second OFDM symbol to the terminal B, and transmission of a first OFDM symbol to the terminal C.
Further, as shown in FIG. 17, it is assumed that timing (hereinafter, referred to as “control update timing”) for updating control of the modulation scheme and transmit power is set to be different timing between adjacent cells. When the control update timing of the modulation scheme and the like is the same as that in an adjacent cell, actual interfering power cannot be reflected in control, and this is because of avoiding an event that the control becomes unstable. The case is described herein where time t4 is set as the control update timing for the terminal A, time t5 and time6 are respectively set as the control update timings for the terminals B and C, and time t7 is set as the second control update timing for the terminal A.
Described herein is a method of determining a modulation scheme and the like of each subcarrier in each base station apparatus 20. FIG. 18 contains graphs to explain the method of determining a modulation scheme and the like of each subcarrier in each base station apparatus 20. It is assumed that FIG. 18 shows the case of using four modulation schemes, 64QAM, 16QAM, QPSK and BPSK.
FIG. 18(a) shows the interfering power level estimated by the mobile station apparatus 30 in the cell 10, and FIG. 18(b) shows the transmission spectrum from the base station apparatus 20 to the mobile station apparatus 30 determined corresponding to the interfering power level estimated by the mobile station apparatus 30. In addition, in FIG. 18(a), the vertical axis represents the interfering power, and the horizontal axis represents the subcarrier number. Meanwhile, in FIG. 18(b), the vertical axis represents the transmit power, and the horizontal axis represents the subcarrier number.
The dashed lines as shown in FIG. 18(a) show maximum allowance interfering power levels to select 64QAM, 16QAM, QPSK and BPSK as a modulation scheme respectively from the below. In other words, 64QAM is selected as a modulation scheme for a subcarrier with the interfering power being the maximum allowable interfering power level for 64QAM or less, and among remaining subcarriers, 16QAM is selected as a modulation scheme for a subcarrier with the interfering power being the maximum allowable interfering power level for 16QAM or less. Similarly, among remaining subcarriers, QPSK is selected as a modulation scheme for a subcarrier with the interfering power being the maximum allowable interfering power level for QPSK or less, and further, among remaining subcarriers, BPSK is selected as a modulation scheme for a subcarrier with the interfering power being the maximum allowable interfering power level for BPSK or less. In addition, a subcarrier with the interfering power level more than the maximum allowable interfering power level for BPSK is set as a carrier hole. FIG. 18(b) shows modulation schemes selected corresponding to the interfering power levels as shown in FIG. 18(a). In addition, types of shading according to the modulation schemes as shown in FIG. 18(b) are the same as in FIG. 15(b). Thus, the base station apparatus 20 determines the modulation scheme and the like at the communication start timing in each mobile station apparatus 30, while updating the modulation scheme and the like at the control update timing, and is thereby capable of selecting a suitable modulation scheme and the like corresponding to the interfering power level estimated by each mobile station apparatus 30 to communicate.
Non-patent Document 1: Toshiyuki NAKANISHI, Seiichi SAMPEI, Norihiko MORINAGA, “Study on interference reduction technique in a one-cell reuse OFDM/TDMA system using a subcarrier adaptive modulation scheme”, IEICE Technical Report RCS2002-239, pages 59-64, 2002